I don’t understand why’s the ML that mad about her “using him” I mean ultimately i...

I don’t understand why’s the ML that mad about her “using him” I mean ultimately it’s in your favor she’s helping your plan he’s just mad because she “doesn’t” feel the same and acting like it’s only sex for her but what I don’t understand is how is having sex helping him? Is she sharing the power of her eyes or whatever that poison the priest gave her must be consumed by her then transmitted to him by sex or something that’s where I’m confused